Stability compensated broadband source and fiber interferometer

Abstract

The present invention discloses a thermally stable rare-earth doped fiber source comprising an active medium such as Erbium or Neodymium. The thermal stability of the mean wavelength of such a source is determined by three contributions as expressed by the following differential equation: ##EQU1## The first term is the intrinsic temperature dependence of the active medium, the second term is the pump power dependence and the third term is a contribution that arises from the dependence of the emission wavelength on the pump wavelength. The method of the present invention minimizes the temperature dependence on the mean wavelength by using the above equation and optimizing the values of the pump power and the pump wavelength so that the three contributions in the governing equation cancel each other.

We claim: 
     
       1. A broadband light source having an emission spectrum, said emission spectrum being characterized by a mean wavelength, said source comprising an active medium which is pumped at a pump wavelength by a pump source to cause said active medium to emit radiation, said pump source having a pump power and a pump wavelength which substantially minimize the sum of the intrinsic temperature dependence of the active medium, the pump power dependence of the mean wavelength and the pump wavelength dependence of the mean wavelength, said sum being defined as: ##EQU11## wherein <λ> is the means wavelength of the signal; T is the temperature;   P is the pump power of the pump source; and   λ p  is the wavelength of the light emitted by the pump source.   
     
     
       2. A broadband source as claimed in claim 1, wherein said pump source is a laser diode. 
     
     
       3. A broadband source as claimed in claim 1, wherein said active medium comprises a single-mode fiber doped with lasing material. 
     
     
       4. A broadband source as claimed in claim 3, wherein said lasing material includes Erbium. 
     
     
       5. A broadband source as claimed is claim 3, wherein said lasing material includes Neodymium. 
     
     
       6. A broadband source as claimed in claim 1, wherein the pump wavelength and the pump power are selected so as to obtain an extremum of the function <λ s  >=λ (T, P, λ p ). 
     
     
       7. A broadband source as claimed in claim 1, wherein the pump wavelength and the pump power are selected such that one of the terms of the governing equation is zero, and the sum of the other two terms is as small as possible 
     
     
       8. A broadband source as claimed in claim 7, wherein the pump wavelength and the pump power are selected such that said other two terms are as close to zero as possible 
     
     
       9. A broadband source as claimed in claim 7, wherein the pump wavelength and the pump power are selected such that said other two terms are as substantially equal and of opposite signs 
     
     
       10. A broadband source as claimed in claim 1, wherein the pump wavelength is selected to correspond to a peak pump absorption rate of the pump source. 
     
     
       11. A broadband source as claimed in claim 1, wherein the pump wavelength is selected to be close but not equal to a peak pump absorption rate wavelength. 
     
     
       12. A broadband source as claimed in claim 10, wherein said peak pump absorption rate wavelength is near 980 nm. 
     
     
       13. A broadband source as claimed in claim 1, wherein the pump wavelength and the pump wavelength are selected so that the total variation of the mean wavelength with respect to temperature is less than 10 ppm/°C. 
     
     
       14. A broadband source as claimed in claim 13, wherein the pump wavelength and the pump wavelength are selected so that the total variation of the mean wavelength with respect to temperature is approximately 1 ppm/° C. 
     
     
       15. A broadband source as claimed in claim 1, further comprising an optical fiber having an input and an output ends and having at least a reflector positioned proximate to said input end of said optical fiber. 
     
     
       16. A broadband source as claimed in claim 15, wherein said reflector is substantially reflective to said emitted radiation and substantially transmissive to said pump light. 
     
     
       17. A broadband source as claimed in claim 15, wherein said reflector is substantially transmissive to said emitted radiation and substantially reflective to said pump light. 
     
     
       18. A broadband source as claimed in claim 15, wherein said reflector is a dichroic mirror. 
     
     
       19. A broadband source as claimed in claim 1, further comprising a backward single pass configuration. 
     
     
       20. A broadband source as claimed in claim 1, further comprising a forward single pass configuration. 
     
     
       21. A broadband source as claimed in claim 1, further comprising a backward double pass configuration. 
     
     
       22. A broadband source as claimed in claim 1, further comprising a forward single pass configuration. 
     
     
       23. A broadband source as claimed in claim 1, further comprising a resonant fiber laser. 
     
     
       24. A broadband source as claimed in claim 1, further comprising a wavelength swept fiber laser. 
     
     
       25. An optical sensor for sensing an ambient effect comprising: a loop comprising an optical fiber having two polarization modes; and   a broadband light source for introducing light into said loop, said source having an active medium which emits radiation in an emission spectrum in response to application of pump energy to said active medium, said active medium being pumped at a pump wavelength by a pump source to cause said active medium to emit radiation, said pump source having a pump power and a pump wavelength which substantially minimize the sum of the intrinsic temperature dependence of the active medium, the pump power dependence of the mean wavelength and the pump wavelength dependence of the mean wavelength, said sum being defined as: ##EQU12## wherein <λ s  > is the means wavelength of the signal;   T is the temperature;   P is the pump power of the pump source; and   λ p  is the wavelength of the light emitted by the pump source.   
     
     
       26. An optical sensor for sensing an ambient effect as claimed in claim 25, wherein said ambient effect is rotation. 
     
     
       27. An apparatus, comprising: an interferometer;   a pump light source that emits pump light and   a broadband light source having an emission spectrum, said emission spectrum being characterized by a mean wavelength, said source comprising an active medium, said active medium being pumped at a pump wavelength by said pump source to cause said active medium to emit radiation, said pump source having a pump power and a pump wavelength which substantially minimize the sum of the intrinsic temperature dependence of the active medium, the pump power dependence of the means wavelength and the pump wavelength dependence of the mean wavelength, said sum being defined as: ##EQU13##  wherein <λ s  > is the mean wavelength of the signal;   T is the temperature;   P is the pump power of the pump source; and   λ p  is the wavelength of the light emitted by the pump source.   
     
     
       28. The apparatus as claimed in claim 27, wherein said interferometer comprises a Sagnac interferometer. 
     
     
       29. The apparatus as claimed in claim 27, wherein said active medium comprises an Erbium-doped single mode optical fiber. 
     
     
       30. The apparatus as claimed in claim 27, wherein said active medium comprises a Neodymium-doped single mode optical fiber. 
     
     
       31. A method for stabilizing the temperature dependence of a broadband source comprising an active medium and having an emission spectrum, comprising the steps of: pumping an active medium by means of a pump source at a pump wavelength to cause said active medium to emit radiation, said emission spectrum being characterized by a mean wavelength; and   selecting the pump power and the pump wavelength of said pump source so as to minimize the total variation of the mean wavelength with respect to temperature, said pump power and said pump wavelength minimizing the sum of the intrinsic temperature dependence of the active medium, the pump power dependence of the mean wavelength and the pump wavelength dependence of the mean wavelength, said sum being defined as: ##EQU14##  wherein <λ s  > is the mean wavelength of the signal;   T is the temperature;   P is the pump power of the pump source; and   λ p  is the wavelength of the light emitted by the pump source.   
     
     
       32. Method for stabilizing the temperature of a broadband source as defined in claim 31, further comprising the step of selecting the active material of the active medium among the rare earth group. 
     
     
       33. Method for stabilizing the temperature of a broadband source as defined in claim 31, further comprising the step of selecting an optical fiber for supporting the active medium. 
     
     
       34. Method for stabilizing the temperature of a broadband source as defined in claim 31, further comprising the step of selecting a value of the pump wavelength and of the pump power such as to obtain an extremum of the following function <λ s  >=λ (T, P, λ p ). 
     
     
       35. Method for stabilizing the temperature of a broadband source as defined in claim 31, further comprising the step of selecting a value of the pump wavelength and of the pump power such that one of the terms of the governing equation is zero, and the sum of the other two terms is as small as possible. 
     
     
       36. Method for stabilizing the temperature of a broadband source as defined in claim 35, further comprising the step of selecting said pump wavelength and said pump power such that said other two terms are as close to zero as possible. 
     
     
       37. Method for stabilizing the temperature of a broadband source as defined in claim 35, further comprising the step of selecting said pump wavelength and said pump power such that said other two terms are substantially equal and of opposite signs. 
     
     
       38. Method for stabilizing the temperature of a broadband source as defined in claim 31, further comprising the step of selecting the pump wavelength to approximately correspond to a peak pump absorption rate of the pump source. 
     
     
       39. Method for stabilizing the temperature of a broadband source as defined in claim 31, further comprising the step of selecting the pump wavelength to be close but not equal to a peak pump absorption rate wavelength. 
     
     
       40. Method for stabilizing the temperature of a broadband source as defined in claim 38, wherein said peak pump absorption rate wavelength is near 980 nm. 
     
     
       41. Method for stabilizing the temperature of a broadband source as defined in claim 31, further comprising the step of selecting the pump wavelength and the pump wavelength so that the total variation of the mean wavelength with respect to temperature is less than 10 ppm/°C. 
     
     
       42. Method for stabilizing the temperature of a broadband source as defined in claim 41, further comprising the step of selecting the pump wavelength and the pump wavelength so that the total variation of the mean wavelength with respect to temperature is approximately 1 ppm/°C.